For the US house of representatives foreign affairs committee to decide that the killing of Armenians by Ottoman Turks in 1915 constituted genocide, as it did Thursday by a one-vote margin, would be acceptable and even praiseworthy if it were part of a serious historical effort to review all the great atrocities of modern history. But the singling out of Turks for censure, among all the killers of the 20th century, is something quite different. This vote was a triumph of emotion, a victory for ethnic lobbying, and another example of the age-old American impulse to play moral arbiter for the world.

Turkey recalled its ambassador in Washington immediately after the vote, which was broadcast live on Turkish television. The resolution now goes to the full House of Representatives. Given the pull of moneyed politics, and President Obama's unwillingness or inability to bring Congress to heel on this issue, as Presidents Bush and Clinton did, it could pass. That would provoke much anger in Turkey, and might weaken the US-Turkish relationship at the precise moment when the US needs to strengthen it.

In the past few years, Turkey has taken on a new and assertive role in the Middle East and beyond. Turkey can go places, talk to factions, and make deals that the US cannot. Yet it remains fundamentally aligned with western values and strategic goals. No other country is better equipped to help the US navigate through the region's treacherous deserts, steppes and mountains.

Would it be worth risking all of this to make a clear moral statement? Perhaps. What emerged from Washington this week, though, was no cry of righteous indignation. Various considerations, including the electoral power of Armenian-Americans, may have influenced members of Congress. It is safe to surmise, however, that few took time to weigh the historical record soberly and seek to place the Ottoman atrocity in the context of other 20th century massacres.

Two questions face Congress as it considers whether to call the 1915 killings genocide. The first is the simple historical question: was it or wasn't it? Then, however, comes an equally vexing second question: is it the responsibility of the US Congress to make sensitive judgments about events that unfolded long ago? The first question is debatable, the second is not.

Congress has neither the capacity nor the moral authority to make sweeping historical judgments. It will not have that authority until it sincerely investigates other modern slaughters – what about the one perpetrated by the British in Kenya during the 1950s, documented in a devastating study that won the 2006 Pulitzer prize? – and also confronts aspects of genocide in the history of the United States itself. Doing this would require an enormous amount of largely pointless effort. Congress would be wiser to recognise that it does not exist to penetrate the vicissitudes of history or dictate fatwas to the world.

This vote has already harmed US-Turkish relations because it has angered many Turks. If the resolution proceeds through Congress, it will cause more harm. This is lamentable, because declining US-Turkish relations will be bad for both countries and for the cause of regional stability. Just as bad, the vote threatens to upset the fragile reconciliation that has been underway between Turkey and Armenia in recent months.

In this episode is encapsulated one of the timeless truths of diplomacy. Emotion is the enemy of sound foreign policy; cool consideration of long-term self-interest is always wiser. Congress seems far from realising this.